The present invention relates to the field of characterising scattering media.
One important property of scattering media is their anisotropy. Indeed, when particles are deformable and/or anisotropic, there are two possible scenarios. In the first scenario, the particles are oriented and distributed randomly, and the medium comprising these particles remains isotropic on a scale greater than the dimensions of the particles. In this case, the propagation of light through the medium will be substantially identical to that of the randomly distributed isotropic particles. In the second scenario, the particles are oriented in a privileged direction of space. The oriented particles negatively affect the propagation of light in the direction of their orientation and promote it in all other directions. Thus, anisotropy of light propagation can be observed on a large scale, for example when the scattering medium is subject to a sheared or elongating flow.
Methods of characterising a scattering medium are known in the prior art, comprising steps of:                generating at least one incident electromagnetic beam;        focussing said incident electromagnetic beam onto a surface of said scattering medium;        collecting at least one scattered electromagnetic beam corresponding to said at least one electromagnetic beam scattered by said scattering medium;        generating a first image representative of said at least one scattered electromagnetic beam;        processing said image;        characterising said scattering medium based on said process.        
Such a method and a device for implementing this method are, for example, known from U.S. Pat. No. 6,011,626. In this document, a characterisation of an isotropic scattering medium is provided by analysing an image representative of a beam scattered by the scattering medium for several polarisation statuses of an incident beam on the scattering medium to be analysed. The device and the method of this document, however, have the disadvantage of not allowing quick access to the anisotropic incoherent transport induced by the scattering medium, and therefore only apply to static or slowly evolving media.
In fact, since the incident radiation emitted onto the scattering medium is also polarised, the effect of polarising the incident radiation combines with the effect of the anisotropic transport of radiation induced by the anisotropies of the scattering medium and prevents access to the anisotropic transport of radiation induced by the anisotropy of the medium. In particular, the device of the aforementioned document involves calculations on all the coefficients of a Mueller matrix, such as illustrated, for example, in FIG. 15 of the aforementioned document. Thus, the device of this document does not in any case allow access to the anisotropic transport of radiation induced by the anisotropy of a scattering medium.
The present invention aims to solve these disadvantages of the prior art. One aim of the present invention is therefore objectively to measure the anisotropic transport of radiation associated with the anisotropy of a scattering medium. Another aim of the present invention is quantitatively to measure the anisotropic transport of radiation associated with the anisotropy of a scattering medium. Another aim of the present invention is to quantify the degree of anisotropy of a scattering medium.
Another aim of the present invention is to measure the effect of the anisotropic transport of radiation associated with the anisotropy of a scattering medium in situ and in a non-intrusive fashion. Another aim of the invention is to measure the effect of the anisotropic transport of radiation associated with the anisotropy of the scattering medium without requiring the use of a polarisation analyser. Another aim of the invention is to define the anisotropic axes of an anisotropic scattering medium.
At least one of these aims is achieved by the present invention, which relates to a method of characterising a scattering medium comprising the following steps:                generating at least one incident electromagnetic beam;        focusing said incident electromagnetic beam onto a surface of said scattering medium;        collecting at least one scattered electromagnetic beam corresponding to said at least one electromagnetic beam scattered by said scattering medium;        generating a first image representative of said at least one scattered electromagnetic beam;        processing said image;        characterising said scattering medium based on said process, characterised in that        said first image is representative of an unpolarised signal associated with said at least one scattered electromagnetic beam;        said processing step comprises sub-steps consisting of:                    determining data representative of an angular variation of said first image;            generating a second image representative of a non-isotropic part of said first image, said second image being calculated using said first image and said data representative of said angular variation; andin that                        said characterisation step comprises sub-steps consisting of:                    characterising the anisotropy of said scattering medium with the help of said second image.                        
Thus, according to the invention, the processes on the electromagnetic beam scattered by the scattering medium are carried out for an unpolarised signal. In this way, only the anisotropic transport of radiation induced by the scattering medium is obtained in the characterisation according to the invention. According to the invention, the data representative of the angular variation of the first image representing the unpolarised scattered radiation is representative of the purely isotropic part of the scattering. This angular variation depends in particular on the nature of the medium and on its concentration, but not on its anisotropy. Having obtained this purely isotropic part, it is then possible, according to the invention, to calculate a second image representative of the non-isotropic part of the scattering. This non-isotropic part represents the anisotropic transport of radiation induced by the medium at the moment of scattering.
The data representative of the angular variation of the first image is, for example, an angular average of the first image, or an angular standard deviation. It should be noted that, in this way, unlike in the method described in U.S. Pat. No. 6,011,626, it is not necessary to use a polarisation analyser at the output of the scattering medium, since the processes according to the invention are performed directly on an image representative of the scattered radiation.
In one embodiment which makes it possible to calculate the non-isotropic part of the scattering, said second image is calculated by the difference between said first image and said data. In one embodiment which makes it possible to calculate the isotropic part of the scattering, said processing step comprises steps consisting of:                determining the baric centre of said first image;        determining said data representative of an angular variation of said first image based on said baric centre.        
In order to obtain a first image representative of an unpolarised signal corresponding to said at least one scattered electromagnetic beam, in the aforementioned method,                said step of generating at least one electromagnetic beam comprises steps consisting of:                    generating a first incident electromagnetic beam having a first polarisation;            generating a second incident electromagnetic beam having a second polarisation, said second polarisation being opposite to said first polarisation;                        said step of collecting at least one electromagnetic beam scattered by said scattering medium comprises steps consisting of:                    collecting a first scattered electromagnetic beam corresponding to said first incident beam scattered by said scattering medium;            collecting a second scattered electromagnetic beam corresponding to said second incident beam scattered by said scattering medium;and wherein said first image is representative of an unpolarised signal corresponding to said first scattered electromagnetic beam and to said second scattered electromagnetic beam.                        
In order to simulate an unpolarised scattering beam using polarised incident radiation, the aforementioned method comprises steps consisting of:                generating a third image representative of said first scattered electromagnetic beam;        generating a fourth image representative of said second scattered electromagnetic beamwherein said first image is equal to the half-sum of said third image and said fourth image.        
The invention also relates to a device for characterising the anisotropy of a scattering medium comprising:                at least one source of electromagnetic radiation capable of generating at least one incident electromagnetic beam;        focussing means capable of transmitting said incident electromagnetic beam onto a surface of said scattering medium;        collection means capable of collecting at least one scattered electromagnetic beam corresponding to said at least one electromagnetic beam scattered by said scattering medium;        generation means capable of generating a first image representative of said at least one scattered electromagnetic beam;        processing means capable of processing said first image;        characterisation means capable of characterising said scattering medium, wherein        said first image is representative of an unpolarised signal associated with said at least one scattered electromagnetic beam;        said processing means comprise processing sub-units capable of:                    determining data representative of an angular variation of said first image;            generating a second image representative of a non-isotropic part of said first image, said second image being calculated using said first image and said data; andwherein                        the characterisation means comprise sub-units capable of:                    characterising the anisotropy of said scattering medium with the help of said second image.                        
According to one embodiment of the aforementioned device, it comprises                a source of radiation capable of generating an initial electromagnetic beam;        a first polariser capable of polarising said electromagnetic beam so as to generate a first incident electromagnetic beam having a first polarisation;        a second polariser capable of polarising said initial electromagnetic beam so as to generate a second incident electromagnetic beam having a second polarisation, said second polarisation being opposite said first polarisation; wherein,        said collection means comprise a collection unit capable of                    collecting a first scattered electromagnetic beam corresponding to said first incident beam scattered by said scattering medium;            collecting a second scattered electromagnetic beam corresponding to said second incident beam scattered by said scattering medium;and wherein said first image is representative of an unpolarised signal corresponding to said first scattered electromagnetic beam and to said second scattered electromagnetic beam.                        
Finally, in order to simulate an unpolarised scattered beam, the aforementioned device comprises an arithmetic unit capable of:                generating a third image representative of said first scattered electromagnetic beam;        generating a fourth image representative of said second scattered electromagnetic beamand said first image is equal to the half-sum of said third image and said fourth image.        